PHOENIX, ARIZONA—A canal dug by the Hohokam people some 1,500 years ago has been unearthed at a construction site for a commuter train station at Sky Harbor airport. The Hohokam built canals to carry water from the Salt River to their irrigated field systems. “We’ve known the fields have been out there. This is the first time we’ve actually seen them,” said Laurene Montero, archaeologist for Phoenix.